1. Field of the Invention
The present invention relates to textile substrates, in particular fibrous nonwoven webs, which exhibit self-cleaning properties, i.e., the so-called lotus effect.
2. Discussion of the Background
There is wide industrial interest in making and using self-cleaning surfaces for a variety of applications. For instance, textiles comprising Teflon® (Gortex®) have the desirable properties of being pervious to water vapor (perspiration) but impervious to liquid water (rain). Textiles, in particular sports and work wear composed of such materials, are now widely used. However, and importantly, mere hydrophobicization of surfaces does not provide surfaces with self-cleaning properties (lotusing).
The principle of self-cleaning surfaces was pioneered in nature by the lotus plant. For a surface to be efficiently self-cleaning it has to have a certain roughness as well as being very hydrophobic. A suitable combination of structure and hydrophobicity ensures that even small amounts of moving water will entrain soil particles adhering to the surface and clean the surface (WO 96/04123).
EP 0 933 388 discloses that self-cleaning surfaces require an aspect ratio of >1 and a surface energy of less than 20 mN/m. Aspect ratio is here defined as the ratio of the height of the structure to its width. The aforementioned criteria are actualized in nature, for example, in the lotus leaf. The surface of the plant, formed from a hydrophobic waxy material, has elevations which are spaced apart by a few μm. Water droplets essentially contact only these peaks. EP 0 909 747 teaches a process for producing a self-cleaning surface.
Production of a self-cleaning surface begins with a surface having hydrophobic elevations 5 to 200 μm high. A dispersion of powder particles and an inert material dispersed in a siloxane solution is applied to the surface. The treated surface is then subsequently cured, thereby resulting in structure-forming particles being immobilized on the surface by an auxiliary medium to form a self-cleaning surface.
WO 00/58410 concludes that it is technically possible to make surfaces of articles artificially self-cleaning. The surface structures necessary for this are composed of elevations and depressions, have a distance in the range from 0.1 to 200 μm between the elevations of the surface structures, and have an elevation height in the range from 0.1 to 100 μm. The materials used for this purpose must consist of hydrophobic polymers or durably hydrophobicized material.
Numerous patent applications are concerned with endowing surfaces with self-cleaning properties. To endow surfaces with self-cleaning properties, particles are frequently applied to the surfaces by means of adhesives. Such processes are utterly unsuitable for finishing textiles, since their flexibility and perviousness to water vapor is lost in the process.
DE 101 18 348 describes polymeric fibers having self-cleaning surfaces wherein the self-cleaning surface is retained by the action of a solvent comprising structure-forming particles, incipiently dissolving the surface of the polymeric fibers by the solvent, adhering the structure-forming particles to the incipiently dissolved surface and removing the solvent. The disadvantage with this process is that processing the resulting polymeric fibers by spinning, knitting, etc may cause the structure-forming particles, and hence the structure responsible for the self-cleaning surface, to become damaged or even completely lost. Should this happen, the self-cleaning effect would be lost as well.
DE 101 18 346 describes textile fabrics having a self-cleaning and water-repellent surface, constructed from at least one synthetic and/or natural textile base material A and an artificial, at least partly hydrophobic surface. The artificial, at least partially hydrophobic surface has both elevations and depressions and comprises particles securely bonded to the base material A without adhesives, resins or lacquers. The textile fabrics having a self-cleaning and water-repellant surface are obtained by treating the base material A with at least one solvent containing particles in undissolved form and subsequently removing the at least one solvent to leave at least a portion of the particles securely bonded to the surface of the base material A.
The last two above-cited processes have the disadvantage that, to obtain secure fixing of the particles to the textile substrates, the particles have to be incorporated in the surfaces of the substrates (or fibers) and this treatment has a deleterious effect on the properties of the substrates (or the fibers).
DE 102 05 007 attempts to overcome this disadvantage by providing for particles suspended in an alcohol to be sprayed onto textile substrates. The alcohol is then subsequently removed. This process avoids damage to the fibers or substrate. Durable fixing of the particles is not possible, however. Thus, while a load generated by falling raindrops is tolerated by the coating, a mechanical load such as scuffing or the like is not.
Thus, known methods of making vapor-pervious, water impervious, and/or self-cleaning substrates, textiles, and fabrics are not satisfactory because the methods weaken the substrates, textiles, and fabrics; render the substrates, textiles, and fabrics incapable of mechanical processing; render the substrates, textiles, and fabrics incapable of remaining intact when put under a load or exposed to mechanical forces, or are complicated and difficult to practice.